The invention relates to the field of injection molding apparatus and provides an injection molding nozzle structure defining a flow path for the heated material wherein an extreme end of the nozzle is configured to break the continuity of a thermally conductive path between the proximal portion of the nozzle, which proximal portion is closer to the heated material supply, and the distal portion of the nozzle, which distal portion bears against the heat dissipating injection mold.
Injection molding is a well known process that involves introducing a heated molten material such as molten plastic resin, into a mold cavity where the material is to cool and set in the shape of an article, the shape being determined by the mold cavity. The set or hardened part is removed from the cavity and the operation is repeated, normally cycling the mold repeatedly to make one part after another.
Among the minimum requirements for injection molding are a source of material that is heated to a temperature higher than the melting point of the material, and means defining a mold cavity that is normally cooler than such melting point and can be opened or accessed so as to remove the cooled article once the article has set in the shape of the cavity.
There are any number of variations as to the articles that are molded, the materials to be used for molding, the structure of the molding apparatus, temperature cycles, flow configurations and other matters. However, the molding material needs to be supplied to the mold in a heated molten state at which the material is liquid or at least viscously deformable, and cooled in the mold to set into the shape of the mold cavity.
Inherently, at least part of the material supply feeding the mold cavity needs to be at a relatively higher temperature, above a softening temperature of the material. Also, the mold cavity needs to be brought at some point in the process to a relatively lower temperature, below such softening point for setting the material.
It is possible to cycle through a range of temperatures. The mold cavity can have a low pre-injection temperature, the cavity being heated by injection of very hot molten material, and carrying away heat energy as the cavity temperature drops to below the material softening temperature. It is inefficient to couple a material supply that needs to be hot, with a mold cavity that needs to be cool. Thermal coupling could cause heat energy associated with melting the material to counteract the need to cool the cavity, and vice versa.
It would be possible to provide thermal insulation as a means to block the passage of thermal energy between a material supply and mold cavity, in any way other than as carried along by the specific heating of the material that moves from one to the other. A thermally insulating valve structure may be applicable. Another technique is to couple the material supply to the mold cavity only intermittently, using a nozzle. The nozzle is in thermal engagement with the material supply, and is hot. The nozzle engages with an injection port leading into the mold cavity during the process of injecting material. After injecting the material, the nozzle is detached, thereby preventing further movement of heat energy between the supply and the mold.
An injection molding coupling could conceivably be male or female on either side to engage with a fitting of the opposite gender on the other side. The coupling can have an associated valve. A heater can be included, particularly in a male fitting or nozzle, associated with the material supply side.
Such an injection molding nozzle (with or without valves or heaters or other particulars) connects the barrel of an injection molding machine (the barrel being the heated material supply) with the mold so as to inject material through the nozzle and into the mold cavity. The nozzle defines the passage for the molten material (or melt) to flow into the mold from the barrel.
The nozzle can be mechanically engaged in a threaded or push-on coupling or the like, if necessary to resist detachment due to the pressure of injection. If the pressure is low, the nozzle can be urged against the corresponding mold cavity port to make the connection of flow paths from the nozzle to the mold cavity. Typically, the melt flow passage in the nozzle is an axial bore and connects to a cylindrical opening in the mold. The passageway in the mold, leading into the mold cavity, is tapered so as to form a larger opening at the mold cavity end and a smaller opening remote from the mold cavity. As a result, material that remains in the passageway after the material sets (known as the sprue) is shaped for ease of detachment and removal from the passageway together with the molded article. The sprue material is the last material to pass from the nozzle into the mold.
In connection with relatively high pressure and high temperature molding operations and in volume manufacturing, exemplified by the molding of polycarbonate data discs for use as audio CDs, CD-ROMs, DVDs and the like, the nozzle is typically made of a durable high temperature metal. The nozzle usually has a spherical shape at the tip and is held against the mold with considerable force. A heater maintains the flowpath through the nozzle at an elevated temperature comparable to the temperature of the material supply or barrel.
The material that remains in the passage leading from the nozzle connection into the mold cavity, namely the sprue, needs to cool together with the article that is formed by hardening of the melt in the mold cavity. The mold cannot be opened to remove the molded article until the article, and the sprue, are cool and stiff enough to be removed without deformation. Removing the article while the sprue is still soft can form an undesirable string of melt material. Sometimes, a still-soft sprue can deform from its own weight to the extent that removal is unreliable or the molded article can be ruined.
The nozzle and the sprue passage are thus problematic. The elevated temperature of the nozzle causes heat to flow from the material supply side toward the cooler mold in the area of the sprue passage. The molten material injected into the mold is naturally hotter near the point of its injection than at remote areas after such material flows into and through the cooler mold during filling. Other things being equal, the material most recently injected into the mold, which unfortunately is the sprue, also is likely to be the hottest material in the mold for a time after the completion of injection. These aspects aggravate the problems associated with the heat energy of the melt material at and adjacent to the sprue and sprue passage, delaying removal of the molded article from the mold and generally causing difficulties that increase molding cycle time or adversely affect the selection rate of production of good parts.